Acoustical concrete



Dec. 4, 1934. a EATO 1,983,021

I ACOUSTICAL CONCRETE I Filed March 11.7, 1931 M W (F IN VENTOR I06. COMPOSITIONS,

COATING OR PLASTIC.

Patented Dec. 4, 1934 UNITED STATES PATENT OFFICE ACOUSTICAL CONCRETE fornia Application March 17,

6 Claims.

This invention relates to the production of an acoustical concrete having a high sound absorptive value and adapted for indoor work, if desired, but more particularly adapted as a wall covering for outdoor work, such as the lining of subways, the construction of outdoor amphitheatres, lining of railway terminals and the like.

Previous attempts to develop acoustic or sound absorbing materials have been confined largely to the interior of auditoriums, radio broadcasting and talking motion picture studios and the like. The demand has existed for a considerable period of time, however, for an acoustical concrete having a high sound absorbing value and yet suitable for outside uses, such as to deaden noises in a railway train shed, in subways and subway platforms and the like. The average concrete which is used in such places has little or no sound absorbing value in that the hard, substantially non-porous surface acts as a reflector for the sound waves striking thereagainst, and the previous types of acoustical plasters cannot be used in such places because of their deterioration on exposure to general outdoor weather conditions and because of their lack of durability to withstand wear and the like.

I have produced, however, an acoustical concrete having many of the usual properties of concrete with reference to hardness, durability, water resistance and the like, and having an inter-cellular structure providing passages for the reception and conduction of the sound waves to the point where they are deadened and ab sorbed by the concrete structure. The acoustical concrete which I have produced has, by actual test, a greater coeflicient of sound absorption than previous specially produced interior acoustical plasters. The concrete may be handled in much the same way as ordinary concrete mix and run into molds or applied with a hawk and trowel and may be given any of the usual exterior finishes which can be applied without bridging or blocking the inter-cellular porous structure.

I have produced a concrete which shows a coefllcient of sound absorption of 40% at 512 double vibrations and which can be applied to any desired thickness without destroying the intercommunicating passages and which shows a proportional increase in sound absorbing efficiency with the increase in thickness.

It is the principal object of this invention to provide an acoustical concrete which has many advantages of usual concrete and which has a high sound absorbing coeflicient much greater 1931, Serial No. 523,273

than normal concrete or similar materials and which may be used for outdoor places or similar places where it is exposed to outdoor weather and moisture conditions without undue deterioration or loss of sound absorptive qualities.

It is another object of this invention to provide a method of forming an acoustical concrete which is highly porous and which is provided with inner communicating pores to increase the sound absorptive efiect.

It is another object of this invention to provide a new product of manufacture which is made of pumice or similar porous material, hide glue and a cementitious binder for the purpose of creating additional intercommunicating pores which increase the sound absorbing eflect.

Further objects and advantages of this invention will appear from the following description thereof, taken in connection with the attached drawing which illustrates the cellular structure of the concrete mix having acoustical properties, the drawing being a representation of a crosssection through acoustical concrete mounted on a common concrete surface or base.

The base 10 is of normal concrete and represents a wall, base or ceiling and may be of any desired thickness for structural purposes. When such a concrete is used in such locations that reflected sound is objectionable, it may desirably be coated with acoustical concrete 12 forming a wall covering which is highly sound absorbent.

The acoustical concrete is a cementitious product having a proportion of ground pumice or other light porous aggregate such as coke breeze mixed with Portland cement. The pumice, of which large deposits occur in California, is of volcanic nature and naturally porous. It is pref- Era erably used in the ratio of six parts of goun? umice with one part of cement such ma. ena s Eemg mixed dry and forming mix A which is thus a dry powder and may be shipped to the job in such condition.

The wet mix, or mix B includes a proportion of hide lue and otash alum mixed with water. A preferred mixiure is' approximately twelve ounces of hide glue together with six ounces of otash al in a proximately one gallon of wa- Eer. The wet mix B is further diluted before use with approximately ten or twelve gallons of water and this amount is suflicient for approxim'atemwo hundred pounds of the dry mix A containing six parts of pumice and one part of cement. The mix will vary however as the demands of the particular job vary, but it will be understood that the pre ent formula although preferred is subject to variations depending on local conditions. The wet mix B, containing the hide glue and alum dissolved in water may be preliminarily made and shipped to the job as is the dry mix A. 5 They are mixed at the job and the additional water is added in such quantities as may be necessary. I

The cement is applied with the usual hawk and trowel in the usual manner of applying plaster and requires no skilled labor or special equipment. The cement may also be run into molds or Wm applied in any other desire manner.

As shown on the drawing, the acoustical concrete 12 has a plurality of interconnecting pores 14 and the lighter colored parts 16 represent the solid part of the concrete. These parts 16 are not aggregate as the concrete is composed of finely ground pumice intermixed with the cement.

20 Due to the glue walls however which entrap air while mix A and mix B are being intermixed and which form bubbles throughout the cement, the

portions of the cement form irregular intercomfmunicating passages as indicated. When the glue walls dry, they shrink, finally breaking and f }thus forming intercommunicating pores through- 'out. Similarly, an irregular surface 18 is formed liand the pores communicate with the depressions 'in this surface.

' The resulting surface coating of acoustical concrete thus has completely intercommunicating pores throughout the coating and these pores are formed by the breaking of the glue walls of the air bubbles. Such a coating, of inch thicki ness, on regular concrete which is almost a perfect sound reflector, shows on test a sound absorption value of 39% at 128 double vibrations and 40% at 512 double vibrations, and nearly at 2048 double vibrations. Such sound ab- 4'0 sorption is substantially higher than any prior acoustical plaster or concrete and yet such coating, being of a cementitious base, is extremely hard, having great durability and a tough surface not affected by water and is therefore suitable I 4 for all outdoor uses or on inside surfaces where 1 conditions of exposure or wear are unduly se- 2 vere. If desired, the acoustical surface may be painted, stenciled, or sprayed with various nonbridging paints or coloring substances without appreciably losing any of its acoustical properties. The surface which is exaggerated on the drawing is substantially smooth although there i are numerous pores, or small surface irregularil ties which receive the sound waves and conduct them into the interconnecting cellular cavities where they are absorbed and not reflected back.

The alum is added to the mix to aid stability. e unus gree of porosity is due largely to the physical properties of the pumice aggregate and the glue used in the mix with a consequent mechanical action which takes place when the material is mixed whereby the glue traps the I air into bubbles which on drying and breaking of the glue walls form interconnecting sound absorbing pores. There is no necessity for using an eifervescent chemical reaction to produce porosity as in mixing the pumice, silica, or coke aggregate the binder being either cement or gypsum, there is a large amount of air naturally entrained in the mixed mortar as on a job this is usually done in a mechanical mixer which thoroughly agitates the mortar. The lightness of the pumice aggregate allows air bubbles to form as the small particles of pumice float around in the mix, and the action of troweling intensif es the mechanical trapping of air which is trapped by the glue walls of the interstices. The fixation of the air by the glue together with the lightness of the material brings about the great porosity which extends through from the surface to the backing on which the acoustical concrete is applied. Such porosity gives to acoustical concrete the unusually high sound absorption qualities which are not characteristic of other known mixtures. The acoustical concrete is particularly adapted for use in outdoor work, lining of subways, construction of arches, stone balustrades, etc., where lightness and reasonable strength is of advantage and where the presence of moisture or other physical conditions require the use of concrete. The thickness of the coat or of any molded form may be varied as desired or as found necessary, and accurate sound absorption can be controlled by varying the thickness of the coats or pieces.

While I have described a preferred formula and process for producing an acoustical concrete I am aware that certain modifications may be made thereto within the scope and spirit of this invention and I therefore desire a broad interpretation of this invention within the scope and spirit of the description thereof and the claims appended hereinafter.

I claim:

1. A composition of matter consisting of natural volcanic pumice and Portland cement in the ratio of six to one by weight and mechanically mixed with an aqueous solution of a gelatinous material of glue and alum forming temporary air bubbles breaking up on drying.

2. A composition of matter consisting of natural volcanic pumice and Portland cement in the ratio of six to one by weight mixed with an aqueous solution of gelatinous material of glue and alum forming temporary air bubbles and having a coefficient of sound absorption in a thickness of threequarters of an inch of not less than 30% at 512 double vibrations.

3. An acoustic, sound absorbing concrete resulting from the setting of a water gauged compo sition, including commercial Portland cement and ground pumice, and an amount of hide glue in the approximate proportion of 6 ounces for each 6 gallons of water used, corresponding to approximately 6 ounces of hide glue to 100 pounds 125 of dry materials, and a stabilizing agent of approximately one-half of the glue.

4. A dry mix composition that on gauging with water forms a plastic mortar capable of app ication by troweling to walls, ceilings and the like, 130 and which sets to form an acoustic sound absorbing concrete surface, including commercial Portland cement and a ground porous mineral aggregate, and a non-chemically active air bubble retaining product of the glutinous type capable of entrapping air bubbles mechanically mixed therein.

5. A plastic sound absorbing wall concrete which is applied by ordinary hawk and trowel methods and on drying has a cellular porous sound absorptive structure from front to back thereof with intercommunicating pores, consisting of a mixture of hydraulic cement, water, pumice, and glue sufficient when mechanically mixed air bubbles are formed to entrap said air bubbles, and releasing said air bubbles when said concrete is dry, said product having a coemcient of sound absorption in excess of 30% at 512 double vibrations and a stabilizing agent.

6. An acoustic sound absorbing concrete result- 150 I06. COMPOSITIONS,

COATING OR PLASTIC.

Examiner and cement with a ratio of 6 of pumice to 1 of cement for approximately 11 or 12 gallons of water and approximately 12 ounces of glue, and a stabilizing agent of approximately one-half as much as the glue. 0

LESLIE B. EATON. 

